Today various display devices have been researched and developed. Inter alia OELDs have been attracting attention as a promising means for obtaining electroluminescence of high brightness at a low driving voltage. An OELD comprising an organic compound in a thin film formed by vacuum evaporation (see Appl. Phys. Lett., vol. 51, p. 913 (1987)) is among them. The reported EL device has a laminate structure composed of a tris(8-hydroxyquinolinato)aluminum complex (Alq) as an electron-transporting material and an amine compound as a positive hole-transporting material and exhibits markedly improved luminescence characteristics compared with a conventional single layer structure.
It has recently been studied to apply an OELD to high-performance color displays or white light sources. To realize the application it is necessary to improve characteristics of each of blue, green and red OELDs.
A green light emitting device with improved characteristics which utilizes light emission from a tris(ortho-metalated) complex of iridium (III) with 2-phenylpyridine [Ir(ppy)3] has been reported in, e.g., Appl. Phys. Lett., Vol. 75, 4 (1999). The device reported achieves an external quantum efficiency of 8%, which is higher than the 5% level that had been regarded as an upper limit that could be reached by predecessors. However, since the light emitted from the device is limited to green light, applicability as a display device is narrow. It has therefore been demanded to develop a multicolor light-emitting device having high efficiency.
A device containing a platinum porphyrin complex is known to give red emission as described in Nature, Vol. 395, 151 (1998) and U.S. Pat. No. 6,303,231B1 but needs improvements with respect to the maximum luminance.
An organic electroluminescent element such as described in U.S. Pat. No. 6,420,057, which comprises a light emission layer containing an organic metal complex having both of an ionic coordinate bond formed between a nitrogen anion and a metal cation and a coordinate bond formed by a nitrogen atom or a chalcogen and a metal is known.